The present invention relates to a process for producing 2,5-dichlorotoluene which is used as a monomer in the production of heat-resistant polymers represented by polyether sulfone, polyether ketone, polyphenylene sulfide, etc. and can be used as a raw material for various organic synthetic chemical substances, for example, medicines and agricultural chemicals.
The present invention also relates to a specific process for producing 2,5-dichlorotoluene in a favorable yield, in spite of the difficulty obtaining 2,5-dichlorotoluene by ordinary chlorination of toluene.
It has been known that various isomers are formed when dichlorotoluene is produced by chlorinating toluene. Namely, since a methyl group possessed by toluene and a chlorine atom which is to be introduced into toluene nucleus by chlorination are the ortho-para orientating groups and the methyl group shows a stronger ortho-para orientation, when toluene is chlorinated to obtain dichlorotoluene, both 2,4-dichlorotoluene and 2,6-dichlorotoluene are easily formed, however, 2,5-dichlorotoluene is relatively seldom formed.
Accordingly, various processes have been proposed so as to produce 2,5-dichlorotoluene. For instance, one prior process involves the preliminary formation of 2-chlorotoluene and then, in step, of introducing the second chlorine atom into 2-chlorotouene, by bringing 0.5 to 0.9 molecule of chlorine into reaction per molecule of 2-chlorotoluene in the presence of a metal catalyst accompanying a sulfur compound as a co-catalyst (see, U.S. Pat. No. 4,031,146). However, in this process, the reaction rate of 2-chlorotoluene must be controlled and sufficiently low, i.e. 50 to 70%, in order to suppress the formation of trichlorotoluene by product in amounts less than 6%, and as a result, there is a problem that the yield of 2,5-dichlorotoluene is inevitably low (the yield being 30 to 42%).
Another process is known whereby 0.9 to 1.1 molecules of chlorine are brought into reaction per molecule of 2-chlorotoluene in the presence of sulfur and/or a sulfur compound as a catalyst (see, Japanese Patent Application Laid-Ooen (Kokai) No. 51-143627/1976). However, in this process, although the yield of 2,5-dichlorotoluene is improved to 50%, there is a problem, on the other hand, that by-production of trichlorotoluene increases to 8%.
Still more, among the processes using 2-chlorotoluene as a starting material, a process of bringing 1.0 to 1.6 molecules of chlorine into reaction per molecule of 2-chlorotoluene while using iodine as a catalyst has been known (refer to Japanese Patent Application Laid-Open (Kokai) No. 57-91934/1982). In this process, although the yield of 2,5-dichlorotoluene is as high as 53%, there is still a problem that as much as 14 to 22% of trichlorotoluene is formed as a by-product.
Moreover, other than these processes, a process of chlorinating 2-chlorotoluene using L-type zeolite as a catalyst has been proposed. However, the selectivity of forming 2,5-dichlorotoluene of the process is only around 56% and a by-production of trichlorotoluene is unavoidable.
As has been described above, all the processes already known are not yet satisfiable for producing 2,5-dichlorotoluene at a high selectivity and in a high yield.
The present invention overcomes many of the problem associated with prior processes described above and thus is capable of producing 2,5-dichlorotoluene at a high selectivity and in a high yield. It has been found that these advantages can be attained by the preliminary introduction of a specified group at the para-position of toluene, chlorinating the treated toluene under specified conditions, and then subjecting the chlorinated compound to transalkylation together with toluene.